Optical spectroscopy allows insights in the structure of matter on an atomic scale. While linear spectroscopy is predominantly used for observing static states, non-linear, time-resolved methods are suitable for examining dynamical processes. However, the well-established time-resolved techniques such as transient absorption spectroscopy and transient grating spectroscopy are limited to the measurement of quantum mechanical populations, while the underlying reason for dynamical changes of the quantum mechanical system, namely the coupling between quantum mechanical states, are not accessible via such spectroscopic methods.
This deficiency is overcome by multi-dimensional spectroscopy, and in particular by the so-called coherent two-dimensional (2D) spectroscopy, to which specific reference will be made in the following. By systematically varying the time delays between three excitation laser pulses and the complete measurement of the non-linear optical response direct information with respect to the couplings between quantum states can be obtained, and this for example allows to determine energy transfer processes in complex systems with a spatial resolution in the nanometer regime and a time resolution in the femtosecond regime.
The results of such a measurement may be arranged in intuitively understandable 2D spectra, in which the emission frequency of the system is plotted versus the original excitation frequency. The intensities of this two-dimensional spectrum which lie off the diagonal axis, i.e. intensities for which the excitation and emission frequencies are different from each other are indicative of couplings and transfer between individual energy states. Using 2D spectroscopy, one obtains the complete spectroscopical information up to third order, and all conventional spectroscopy techniques such as transient absorption etc. are automatically included therein.
While 2D spectroscopy is by far the most common kind of multi-dimensional spectroscopy, it is nevertheless possible to use a higher number of excitation pulses and to generate higher dimensional spectra. For example, five excitation pulses have been used in order to generate a fifth order signal. For simplicity, the present specification focuses on the case of 2D spectroscopy, but it is to be understood that the principles discussed herein can equally be employed for higher dimensional spectroscopy.
An example illustrating such kind of 2D spectra is shown in FIG. 1, which has been taken from T. Brixner, J. Stenger, H. M. Vaswani, M. Cho, R. E. Blankenship, and G. R. Fleming. “Two-dimensional spectroscopy of electronic couplings in photosynthesis”, NATURE, 434:625-629, March 2005. Diagrams a, b and c of FIG. 1 show the 2D spectra of the Fenna-Matthews-Olsen (FMO)-Bateriochlorophyll-a-protein of green sulphur bacteria, which serves both, as an antenna molecule for harvesting light energy as well as a mediator for guiding light excitations from the chlorosome antenna to the reaction centre. The horizontal axis in the diagrams a to c of FIG. 1 corresponds to the absorption frequency ωt, and the vertical axis corresponds to the emission frequency ωt. The 2D spectra of diagrams a to c correspond to population times T of 0 femtoseconds (fs), 200 fs and 1000 fs, respectively, where the “population time” is the time between the second and third excitation light pulses.
Diagram d shows the experimentally determined linear absorption spectrum (solid line) as compared with its theoretically determined counterpart as well as exciton contributions (chain-dotted lines). Diagrams e and f show simulations of 2D spectra for population times of T=200 fs and T=1000 fs, respectively. In the 2D spectra of FIG. 1, intensities A and B can be seen which do not lie on the diagonal axis and which are therefore indicative of couplings between quantum states.
The reason for the off-diagonal intensities A and B, the so-called “cross-peaks” is that the structural components of the macromolecule, to which peaks A and B correspond, are aware of each other. This means that the structural components are so close to each other that they are quantum mechanically coupled, and the pulse sequence induces transitions therebetween. To be precise, the intensity in the 2D spectrum corresponds to the probability that a photon having a frequency ωt is absorbed and is re-emitted, after a population time T, at a frequency ωt. This type of information can not be discerned from a one-dimensional spectrum, and this illustrates the unique information provided by 2D spectra.
Optical 2D spectroscopy is in many respects analogous to 2D nuclear magnetic resonance, which has become an inevitable indispensable standard method for structure analysis of molecules and which is employed in practically any chemical analysis laboratory using commercially available apparatuses. “Nuclear magnetic resonance is based on couplings of nuclear spins of single atoms and accordingly reflects their molecular structure. As can be seen from the above description of FIG. 1, optical 2D spectroscopy is sensitive for couplings of complete chromophores, i.e. couplings of larger molecular units or functional groups and accordingly reflects the structure of supramolecular configurations, which are relevant for a number of important technical and scientific applications, for example applications in the field of organic photovoltaics, natural and artificial photosynthesis complexes, quantum dot systems, pigment complexes, aggregated π-complexes etc.
For a comprehensive explanation of the fundamental principles of coherent two-dimensional optical spectroscopy, reference is made to the review article “Two-dimensional femtosecond spectroscopy” by David M. Jonas, Annual Ref. Phys. Chem. 2003, Vol. 54, 425-463 as well as review article “Coherent two-dimensional optical spectroscopy” by Minhaeng Cho, Chem. Rev. 2008, 108: 1331-1418 and the references cited therein.
Ever since the pioneer works of Hamm et al. (S. Woutersen and P. Hamm. “Structure determination of trialanine in water using polarization sensitive two-dimensional vibrational spectroscopy”, JOURNAL OF PHYSICAL CHEMISTRY B, 104:11316-11320, November 2000), Hochstrasser et al. (P. Hamm, M. H. Lim and R. M. Hochstrasser. “Structure of the amide i band of peptides measured by femtosecond nonlinear-infrared spectroscopy”, JOURNAL OF PHYSICAL CHEMISTRY B, 102:6123-6138, July 1998) and Tokmakoff et al. (M. Khalil, N. Demirdöven and A. Tokmakoff, “Coherent 2D IR spectroscopy: Molecular structure and dynamics in solution, JOURNAL OF PHYSICAL CHEMISTRY A, 107:5258-5279, July 2003), coherent optical 2D spectroscopy in the IR spectral range can be regarded as an established method. FIG. 2 schematically shows the setup of a 2D experiment which has been taken from the above referenced publication of Tokmakoff et al. In FIG. 2, the first, second and third excitation pulses are referenced at α, β and χ, respectively. The individual beams are generated from the original incoming beam using beam splitters, which are referenced in FIG. 2 as “BS”. In addition, a fourth beam is generated which is referenced as “LO” and represents a local oscillator signal. As will be explained in more detail below, the LO-signal is superposed with a third order signal resulting from an interaction of the first, second and third pulses with the sample, to allow for a heterodyne detection. The timing of the first to third pulses and the LO-pulse can be adjusted using ordinary delay lines comprising movable mirrors.
While the set up of FIG. 2 is designed for IR spectroscopy, many interesting systems require spectroscopy in the visible spectral range (500 nm-750 nm). This is for example true for biological systems, organic solar cells and artificial photosynthesis complexes. For such short wave lengths, the setup of FIG. 2 is not suitable, since any variation in the optical wavelength, which can not be avoided with the setup of FIG. 2 will lead to a ten times higher phase error since the wavelength is ten times shorter as compared to IR, thus introducing errors to the signals. In other words, the setup of FIG. 2 which in case of IR-spectroscopy in many cases provides a satisfactory phase stability can no longer provide sufficient phase stability for light pulses in the visible optical range. Nevertheless, even in the IR-range phase stability is a significant technical problem, which can only be solved with considerable technical efforts, for example using an active phase stabilizing technique based on a control loop.
In 2004, optical spectroscopy has been further developed by Brixner et al. (T. Brixner, I. V. Stiopkin and G. R. Fleming, “Tunable two-dimensional femtosecond spectroscopy”, Optics Letters 29, 884 (2004) and T. Brixner, T. Mancal, I. V. Stiopkin and G. R. Fleming, “Phase-stabilized two-dimensional electronic spectroscopy”, Journal of Chemical Physics 121, 4221 (2004)) and Miller and al. (M. L. Cowan, J. P. Ogilvie and R. J. D. Miller, “Two-dimensional spectroscopy using diffractive optics based phased-locked photon echoes”, Chemical Physics Letters 386, 184 (2004)), such that wavelengths in the visible spectrum could be used.
The setup used by Miller et al. employs diffractive optics, namely an optical grating having a small line density at which two incoming beams are split into a total of four beams, where two of the beams are phase-stable with respect to each other. Time delays are introduced using conventional delay lines and retro-reflectors, or alternatively using rotatable glass plates.
The setup of Brixner et al. corresponds to an apparatus according to the preamble of claim 1 and is schematically shown in FIG. 3. This setup too uses a diffractive optic. However, instead of conventional delay lines, pairs of movable glass wedges are used in order to precisely introduce delays. As can be seen in FIG. 3, two parallel partial beams generated via a beam splitter (not shown) are focused via a lens onto a grating (“diffractive optic”). Using the +1st and −1st order of diffraction of this grating four beams are generated, which are focused onto a sample using a spherical mirror (f=25 cm). Downstream of the sample, the three excitation pulses 1-3 are blocked using an aperture, and only the superposition of the third order signal and the local oscillator (i.e. the 4th pulse) reaches the spectrometer.
Since all beams are guided along the same optical elements, this setup is inherently phase-stable. Time delays τ between the first and second light pulses and T between the second and third light pulses are introduced via the aforementioned path through the glass wedges, which can be shifted with respect to each other using stepper motors. This way, the optical wavelength of the individual beams can be varied extremely precisely and in a reproducible manner, such that a nominal precision of 2.7 attoseconds (as) can be obtained. As is demonstrated in the publications cited above, using this setup an excellent phase stability can be obtained even when pulses in the visible spectral range are used.
The main reason for the increased phase stability in this prior art has to do with the use of an optical grating for splitting the beams. Even when the optical grating moves due to unavoidable vibrations, no relative difference in the optical wavelength of the beams split at the grating occurs, so that the beams are inherently phase-stable. Accordingly, for the person skilled in the art of 2D spectroscopy, a passive phase stabilization is synonymous with the use of optical gratings.
Alternative active methods for obtaining a sufficient phase stability and for generating precise time delays between the pulses have been introduced by Nelson et al. (T. Hornung, J. C. Vaughan, T. Feurer and K. A. Nelson, “Degenerate four-wave mixing spectroscopy based on two-dimensional femtosecond pulse shaping”, Optics Letters 29, 2052 (2004)) and by Damrauer and Zanni (E. M. Grumstrup, S.-H. Shim, M. A. Montgomery, N. H. Damrauer and M. T. Zanni, “Facile collection of two-dimensional electronic spectra using femtosecond pulse-shaping technology”, Optics Express 15, 16681 (2007)). In these methods, an active phase modulator is employed in a suitable pulse former. However, such active phase modulator is rather costly, and the construction with finite size optical pixels limits the available time range for 2D spectroscopy.
Finally, according to Zhang et al., phase stability can also be achieved using an active control loop having a feedback mechanism (T. Zhang, C. N. Borca, X. Li and S. T. Cundiff, “Optical two-dimensional Fourier transform spectroscopy with active interferometric stabilization”, Optics Express 13, 7432 (2005)). This requires an additional ongoing measurement of the relative phase positions and a closed loop trying to constantly correct the path lengths such that the measured phase remains constant. This technique is comparatively complicated, and there is a risk that the control loop introduces additional oscillations.
While the experimental setup of FIG. 3 has been generally successful, it would be advantageous to modify this setup such that it may find a broader use. The ultimate aim is that 2D spectroscopy (or the more-dimensional spectroscopy in a more general sense) can not only be carried out in specifically equipped laboratories by experts in the field of ultra fast spectroscopy, but to be generally employed in chemical or biological laboratories for determining the structure and dynamics of supramolecular compounds.
Additional related apparatuses are disclosed in US 2006 0063 188 A1 and WO 2007/064 830 A1.